The present invention relates to a supercharging device for an internal combustion engine for increasing the air-intake pressure of the internal combustion engine, and more particularly, to a supercharging device including a speed increasing gear device of a planetary gear type.
Conventional mechanical driving mechanisms of superchargers for internal combustion engines are disclosed in the International Publications No. W089/02521 (which corresponds to the official announcement Hei 3-500317 (1991) in Japan) and the International Publications No. W089/02535 (which corresponds to the official announcement Hei 3-500319 (1991) in Japan). FIG. 1 illustrates the mechanical driving mechanism disclosed in the former publication. This supercharger 201 is constructed so as to increase the driving force of an engine by employing a planetary gear type of speed increasing gear device and to rotate an impeller shaft 207 of an air compressor.
In this supercharger 201, an oil film damper 215 is formed between a guide sleeve 211 and a casing 213 which support a pair of bearings 209a, 209b of the impeller shaft 207. The oil film damper 215 is necessary for absorbing the vibration of the impeller shaft 207 which rotates at high speeds. However, the impeller shaft 207 is suspended in this oil damper, thereby making the support of a sun gear 217 formed on the impeller shaft 207 unstable. Since the bearings 209a, 209b are angular contact types of ball-and-roller bearings and have larger radii compared to other types of bearings like a sliding bearing, the radius of the guide sleeve 211 must also be larger. Thus, the weight which the oil damper 215 supports becomes large, thereby requiring the oil film to be thick. Consequently, the ill-effects of the oil film damper 215 upon the support of the sun gear 217 is endorsed, thereby causing a bad engaging condition between the sun gear 217 and the pinion gears 219.
Furthermore, a pressure is previously added to the angular contact type of bearings 209a, 209b by the guide sleeve 211. Therefore, if the guide sleeve 211 swells due to the heat from the sun gear 217, etc., the added pressure is changed, thereby resulting in poor durability of the bearings 209a, 209b. Further, the lubricating oil is splashed from the pinion gears 219 toward the sun gear 217, thereby resulting in poor lubrication at times. If the lubrication of the sun gear 217 is inadequate, drying of the lubricating oil due to heat causes further poor lubrication condition.
Meanwhile, the thrust force of the impeller shaft 207 is received by the ball-and-roller bearings 209a, 209b. In order to prevent the synthetic vibration or damage due to synthetic vibration, it is necessary for the natural vibration frequency of the impeller shaft 207 to be sufficiently higher than the synthetic vibration frequency, thereby requiring as large a span between supporting points as possible. Further, the ball-and-roller bearings 209a, 209b have a larger amount of play in the radial direction thereof than those of a sliding bearing of the full-floating type in which oil film is employed. Accordingly, it is difficult in this type of mechanism to accurately widen the span. Further, if the ball-and roller bearings 209a, 209b, which have larger radii than those of a sliding bearing, are arranged in a relatively wide span, interference with the circumferential members occurs and the scale of the entire mechanism becomes large. In addition to these shortcomings, it is difficult for this mechanism to regulate synthetic vibration more effectively than a floating type of sliding bearing.
FIG. 2 illustrates a conventional type of supercharger 208 which has a similar configuration to that disclosed in the Japanese Utility Model application Laid Open No. 58-177529. In this supercharger which is for turbo-charging, an impeller shaft 212 connecting a compressor impeller 210 with a turbine impeller is supported by a bearing holder 218 through a pair of sliding bearings 214, 216.
The sliding bearings 214, 216 are not employed to receive a thrust force as does the ball-and-roller bearings 209a, 209b shown in FIG. 1. Accordingly, a thrust washer 220 is arranged in a groove 222 provided close to the impeller shaft 212 and is supported by the bearing holder 218, to receive the thrust force. The bearing holder 218 is integrally formed so as to support the sliding bearings 214, 216 along the same line. Further, the thrust washer 220 is arranged outside of the sliding bearings 214, 216, specifically, on the compressor side. Therefore, the overhang of the distal part of the compressor impeller 210 over the sliding bearings 214, 216 has to be large, thereby causing vibrations easily and a narrow span between the sliding bearings 214, 216.